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Pettit, EC, Waddington ED, Harrison WD, Thorsteinsson T, Elsberg D, Morack J, Zumberge MA.  2011.  The crossover stress, anisotropy and the ice flow law at Siple Dome, West Antarctica. Journal of Glaciology. 57:39-52.   10.3189/002214311795306619   AbstractWebsite

We used observations and modeling of Sip le Dome, West Antarctica, a ridge ice divide, to infer the importance of linear deformation mechanisms in ice-sheet flow. We determined the crossover stress (a threshold value of the effective deviatoric stress below which linear flow mechanisms dominate over nonlinear flow mechanisms) by combining measurements of ice properties with in situ deformation rate measurements and a finite-element ice flow model that accounts for the effects of viscous anisotropy induced by preferred crystal-orientation fabric. We found that a crossover stress of 0.18 bar produces the best match between predicted and observed deformation rates. For Sip le Dome, this means that including a linear term in the flow law is necessary, but generally the flow is still dominated by the nonlinear (Glen; n = 3) term. The pattern of flow near the divide at Sip le Dome is also strongly affected by crystal fabric. Measurements of sonic velocity, which is a proxy for vertically oriented crystal fabric, suggest that a bed-parallel shear band exists several hundred meters above the bed within the Ice Age ice.

Nooner, SL, Eiken O, Hermanrud C, Sasayawa GS, Stenvold T, Zumberge MA.  2007.  Constraints on the in situ density of CO2 within the Utsira formation from time-lapse seafloor gravity measurements. International Journal of Greenhouse Gas Control. 1:198-214.   10.1016/s1750-5836(07)00018-7   AbstractWebsite

At Sleipner, CO2 is being separated from natural gas and injected into an underground saline aquifer for environmental purposes. Uncertainty in the aquifer temperature leads to uncertainty in the in situ density of CO2. In this study, gravity measurements were made over the injection site in 2002 and 2005 on top of 30 concrete benchmarks on the seafloor in order to constrain the in Situ CO2 density. The gravity measurements have a repeatability of 4.3 mu Gal for 2003 and 3.5 mu Gal for 2005. The resulting time-lapse uncertainty is S.3 mu Gal. Unexpected benchmark motions due to local sediment scouring contribute to the uncertainty. Forward gravity models are calculated based on both 3D seismic data and reservoir simulation models. The time-lapse gravity observations best fit a high temperature for-ward model based on the time-lapse 3D seismics, suggesting that the average in Situ CO2 density is about to 530 kg/m(3). Uncertainty in determining the average density is estimated to be 65 kg/m(3) (95% confidence), however, this does not include uncertainties in the modeling. Additional seismic surveys and future gravity measurements will put better constraints on the CO2 density and continue to map out the CO2 flow. (C) 2007 Elsevier Ltd. All rights reserved.